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Helmantel

Chassis dimensions - measuring & straightening

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Posted (edited)

Hi all, 

I'm working on the body of my 79 SC which I managed to crash a few years ago. The work includes straightening the body at several points, for which I converted my four post lift into a straightening bench. It's quite a task and labor intensive for an amateur like me, but I'm making progress. 

Anyway, my question is about the rear shock absorber mounting upper points. There are various workshop manual drawings you can find on the internet that have a dimension written but the one that applies for a 79 (73-83 workshop manual) has the wrong dimension. Unless either my shock mounting points have moved two inches (which they haven't) or I'm misreading the drawing. When I scale the length of the arrow that indicates the upper shock mount to the horizontal one between transmission and rear engine mounts (1144 mm), the dimension should be ~372 mm, which is a lot closer to dimension I'm measuring. Unfortunately, the drawing is rather blurry and the exact position of the upper measuring point isn't very clear either. 

Does somebody have a better drawing that is more useful? The same drawing but sharper would also be helpful. Note that many of the drawings that can be found online are for the earlier models (shocks angled rearward). Some of those drawings are a little better, but unfortunately of no use. 

Thanks, 

Arjan

image.png.9581d597146b068152dbdfcb4231d556.png

 

Edited by Helmantel

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This is not what you want to read.

Take your car to a Porsche specialist with the the correct jig, mounts and reference points.

As always this is just my opinion but I don’t see any merit in attempting to straighten a bent chassis with anything but the correct tools and more importantly specific model knowledge.

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Sounds like a good project to me - if you know everything is square & level and you have the dimensions then no problem to at least have a measure, especially if you are miles from a decent shop with a jig and brackets. I know a lot of bodyshops use a floor-mounted measuring system and wonder if there might be charts available for that with dimensions. Long shot.

I suppose maybe trying to find someone who is restoring a shell like yours on a jig and getting some measurements would be my first line of enquiry. Once you have a rough idea of how bent it is then you can move on to figuring out how to pull it.

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Well, the original plan was to take it to a specialist but after a few "sorry, not this year" replies I decided to do it myself. 

Here some examples of the jigs and mounts I made. I got the front end pretty much back in place. The first picture shows pulling the left side front 5 mm down and the second one  it being level again and welded to the jig to keep it in place when adjusting the rest.  The first picture also shows the mount for the front pivot points for the control arms (standing up straight, as a support for the jig) 

Now I started on the backside where the most of the damage is . 

I could figure out all the attachment points from the drawings except for the rear shock mounts. So if anyone has a better drawing it would be much appreciated. 


IMG_7455.thumb.JPG.52083c79db9ce37e9405e574589ebbfa.JPGIMG_7462.thumb.JPG.5f758b647cb96c7767499bda3370977c.JPGIMG_7445.thumb.JPG.8cf50a8d2a5e6cc46a3c878db512a6c6.JPG

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I'll hopefully have the masking off my '79 targa shell in a couple of days.  My rear shox are in but as the engine and box are out, I can try and get some measurements for you if still needed.

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Thanks! I realize that I can't be picky about the quality of the measurement but it would need to be measured relative to a reference point/line (like the transmission mounting points), with the car level. I'm using a cross line laser but I don't know if you have one. 

If you want to try it, then of course I appreciate it but it takes quite some effort and time before you can make a valid measurement. 

image.png.14f8b9df0043be24bd2e1def6ce40c89.png

 

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37 minutes ago, Helmantel said:

Thanks! I realize that I can't be picky about the quality of the measurement but it would need to be measured relative to a reference point/line (like the transmission mounting points), with the car level. I'm using a cross line laser but I don't know if you have one. 

If you want to try it, then of course I appreciate it but it takes quite some effort and time before you can make a valid measurement. 

image.png.14f8b9df0043be24bd2e1def6ce40c89.png

 

If anyone can @PeterK can 👍

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I realized that, after (quickly) browsing through his restoration thread! My operation seems rather modest in comparison :)

 

 

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1 hour ago, Helmantel said:

I realized that, after (quickly) browsing through his restoration thread! My operation seems rather modest in comparison :)

You are doing quality work using your own resources and brain power - great job and keep going!

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2 hours ago, johndglynn said:

You are doing quality work using your own resources and brain power - great job and keep going!

Agree 100%, looks like a lot of thought has gone into getting it back to the right shape

I'd imagine it will be straighter when done than most of the Porsche specialists out there could achieve as not many places have the right jig/equipment or skills

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15 hours ago, Helmantel said:

I realized that, after (quickly) browsing through his restoration thread! My operation seems rather modest in comparison :)

 

 

I think you are doing yourself quite an injustice there.  Good effort for tackling the job; I hope you get it sorted.  The specialists might have nice jigs, but it is still a human and hydraulics doing the work.

I’ll have a look at the body I have in the workshop and see if I can get some data for you.  What I can tell you, knowing someone that has done a lot of 911 work, is that some measurements are non existent which is possibly the issue you have found.  When they did jig work, it was often a case of using rack of eye and making the body fit the replacement panel, so to speak.

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956770849_rearmeasurements.thumb.jpg.88f67fc5d2037ede17c9f845f11c58b6.jpg

Dunno if that comes across any sharper.  The pdf I took it from is no sharper...

Al.

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16 hours ago, Flat 6 said:

 

Dunno if that comes across any sharper.  The pdf I took it from is no sharper...

Al.

That's about as sharp as my pdf, but thanks anyway. Besides, 333,5 mm is about two inches too short, so the drawing is wrong. One possibility is that the distance is to "plan O" instead. This is the reference line across the car and 42 mm above the torsion bar center. 

Thanks for the help anyway, 

@oliverjamesthomas: If it's not too much effort, it would be nice if you had a look

 

 

 

 

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I dropped and broke my laser tool, so I think we'll scratch my offer, but ....

Reverting to a hastily considered Plan B, I balanced a spirit level on my trolley jack, so that the top edge rested against the underside of the gearbox mounting face.  As my shell was just on dollies and blocks of wood now, and no longer level front to back, I checked the body - 1.20' downwards slope back to front.

I set the spirit level to the same slope and measured to my upper spirit level (held across the mid point or each shock turret, and got exactly 440mm !  That seems way away from the factory manual, but that said, doesn't look too far out relative to the 253.5 measurement to the right of the 333.5 measurement.

 

Anyway, a clunky measurement and it was was it was.

Peter

 

IMG_0873.jpeg

IMG_0874.jpeg

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Is this dimension that critical ?
 

The torsion tube outboard ends and gearbox mounts (as a proxy for the inboard ends of the torsion tubes) broadly set the rear suspension, with some minor tweaking to camber, caster & toe possible later.  The shock mountings don’t (within reason) matter, as long as the shox themselves never reach either ends of their travel.

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Looking at the drawing, the 333.5 dim is to a point in the centre of the hole in the larger pressed cap, not the hole at the top of the smaller one on top of it.  Convenient for dimensioning the jig to build the car on, but not much use on the finished car as it's a dimension to a point in fresh air......

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Thanks for the measurement, Peter. Very helpful. 

I just checked mine and found ~442 on the right side and ~435 on the left side. I already knew that they were uneven so that was no surprise. 

You are of course right that the exact height of this point, being the mounting point of a telescopic damper, is not all that critical. 

The reason I started to wonder about this dimension is that the rear cross member was smacked up and the chassis legs bent down with the right side being the worse. I cut through the rear cross member down the middle and jacked up the  right chassis leg. After some checking I found that even though the back end of the right leg (around where the rear engine mounts are located) was still lower than the left one, the right shock mount sat higher. Unfortunately, I didn't check the shock mount positions before I start jacking up at the end.  Then I started to wonder where these points should be, because I wanted to avoid to bent things back and past the point where they should be.  

I'll just make sure that the shock mounts are even on both sides and that the ends of the chassis legs (and rear engine mounts) are in the correct position, then all should be fine. 

Again, thanks for the help. I'll post some updates of the work when I make any. 

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IMG_7500.thumb.JPG.0c5cc07de8593c360111d2133f80607e.JPGIMG_7501.thumb.JPG.37f662c47a891bbce4b755862dbd6b8d.JPG

Small update: I made a mount for the rear shocks. As you can see, it does not sit level so the next step is push and pull until it does. The question was if one side needed to go up  or one side up and the other one down and if so, in equal or different amounts, hence my quest for the position of the upper shock mounts. In the end, the ends of the chassis legs (where the rear engine mounts are located need to be at the correct height and equal and the shock transverse member needs to be level and centered. 

You can also notice that making a body from galvanized sheet metal is a good idea, as long as they're not shaped as a trap for dirt, water and salt (e.g. the kidney beans and various other spots in the wheel wells. BTW (and maybe you knew this), the bodies were welded together from galvanized steel parts, not dipped in a galvanizing bath after being finished (which is also possible). You can see that wherever there are welds (regular, not spot welds), which are all rusty. 

 

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9 minutes ago, Helmantel said:

imageproxy.php?img=&key=26d24dfdaa8bd0ebimageproxy.php?img=&key=26d24dfdaa8bd0ebIMG_7500.thumb.JPG.0c5cc07de8593c360111d2133f80607e.JPGIMG_7501.thumb.JPG.37f662c47a891bbce4b755862dbd6b8d.JPG

Small update: I made a mount for the rear shocks. As you can see, it does not sit level so the next step is push and pull until it does. The question was if one side needed to go up  or one side up and the other one down

 

 

These are not easy body tubs to straighten.
The position of shock towers needs to be measured back from the other reference points - front suspension pickup points, torsion tubes etc. It’s impossible to decide what needs moving without establishing accurate datums.  
You may find that some of the box sections need a strategic cut to allow things to start moving. Localised heat and a very heavy duty key puller will also assist with the positioning and prevent the steel from folding in the wrong direction. 
 

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The other suspension pick up points are all centered and level within 1 mm so that should be OK. But you're right that's it's not easy and every movement of some section influences another one. 

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